1. Technical Field
This invention relates to communications networks, and more particularly to synchronous fiber optic telecommunication networks, specifically to the demarcation and performance monitoring of such networks at critical interfaces between public and private network entities.
2. Background Art
Fiber Optic networks have been in existence for some time now. The original fiber optic networks however typically employed proprietary asynchronous multiplexing schemes and interoperation between unrelated network elements was generally not possible. As fiber optic networks became more prevalent the need to efficiently connect fiber optic network segments directly at the optical level has become more critical.
The Synchronous Optical Network (SONET) standards were developed by the telecommunications industry to overcome the limitations of proprietary asynchronous schemes. SONET standards define an open network architecture that allows true multi-vendor interoperability. The SONET standards have subsequently been adopted by the international community and are known worldwide as the Synchronous Digital Hierarchy (SDH).
An essential element of the SONET standard signal structure is a transport overhead data structure that supports enhanced network performance monitoring and network operations. The transport overhead contains among other things: parity information, protection switching control bytes, payload pointer and control bytes, a user data channel, local and express orderwire channels, and two data communication channels.
The two data communication channels are actually independent packet data networks designed to support Network Operations and Support Systems. Network Operations and Support Systems are the management tools used by telecommunication service providers to control and monitor the various elements present on their networks. Using the CCITT Q.921 LAPD protocol, which is very similar to the ANSI X.25 protocol, it is possible for any device on the network to access and control any other device via the SONET data communication channels.
It was the intent of the SONET standards committees to develop a ubiquitous and homogenous fiber optic network whereby any element of the network could fully interoperate with any other element in the network. Manufacturers of SONET switching systems and SONET multiplexers have been staging multi-vendor demonstrations with an emphasis on interoperability at the fiber optic level.
The SONET standards however do not adequately reflect the political nature of the SONET network. Whereas SONET strives towards universal access and interoperability there are operational factors that force network operators to segment and isolate portions of the network. This situation can occur when two different service providers must interface with each other, or when a public service provider must extend service to a private customer premise.
For obvious reasons relating to network security, service providers need to restrict outside access to internal network operations. The data communications channels contained within the SONET transport overhead, designed to be open network management interfaces, represent a potential weak spot in the network security system.
Presently, SONET network segments are typically isolated at the boundaries between service providers by using back-to-back multiplexers; only the constituent channel data is passed between network segments and the SONET transport overhead functions are removed entirely. This type of interface defeats the open architecture of the network but must be maintained as a well defined demarcation point between network segments.
When extending SONET services to a private customer premise, a public service provider must rely upon the limited network security features supported by the transmission equipment on either end of the fiber optic span to protect the public network operations systems from unauthorized or unintended access. As Asynchronous Transfer Mode (ATM) terminals become more commonplace this problem will become more critical.
There exists a need for a unique device that can be used to interconnect SONET network segments directly at the SONET signal level while selectively isolating and controlling the transport overhead data passing between the segments. This invention, referred to as an Optical Service Unit (OSU), directly addresses this need by combining hardware technology from the SONET multiplexer industry with an embedded software package designed to control and monitor the SONET transport overhead.
There is very little prior art that relates to this application. Beyond the fact that SONET systems are relatively new, the lack of prior art is also related to the novel and unobvious nature of this device which runs counter to the open systems interconnection objectives of the SONET standards. The function represented by the Optical Service Unit is not defined in the SONET standards.
Previously, optical interfaces have differed from the Optical Service Unit because they all terminate proprietary networks. The Optical Service Unit however acts as an mid-span intelligent interface between two standardized networks; the Optical Service Unit passes only the payload plus selected overhead data as programmed by the user.
The Optical Service Unit includes an orderwire function similar to that described by U.S. Pat. No. 4,449,247 to Waschka, but the implementation is based upon the embedded orderwire channels in the SONET transport overhead. In contrast, Waschka use a wavelength multiplexing technique on a proprietary signal format.
Some traditional T1 and T3 electrical network interfaces contain a device referred to as a Channel Service Unit, from which the Optical Service Unit derived its name. The Channel Service Unit terminates and isolates a digital trunk interface in order to protect the public network from harm due to improper voltage levels or signaling formats that may be introduced by the customer premise equipment. Channel Service Units can also be configured to monitor network performance and support basic network testing operations.
The electrical network however was never intended to be as fully open and interoperable as is the SONET network. Channel Service Units perform the very clear and well defined function of isolating the electrical network at its demarcation points. The Optical Service Unit will perform a similar function for the optical network however the task is far more complicated.
Unlike a Channel Service Unit, the Optical Service Unit cannot totally isolate the SONET network. If it did, the benefits of having an extended network management system via the transport overhead would be lost. The Optical Service Unit must intelligently pass the transport overhead; allowing some but not all of the overhead data to pass.
The filtering function of the Optical Service Unit is not unlike that found in Local Area Network Routers. Routers examine the data terrific on the Local Area Network and pass only that data which meets a user defined set of criteria. Routers are commonly used to segment Local Area Networks for maintenance, performance, and security reasons.
In this invention the functions of the Local Area Network Router are merged with the functions of a channel service unit; the product is then embedded in the hardware technology of a SONET Multiplexer front-end to create an Optical Service Unit. This novel combination of features has been overlooked by industry and no similar device is known to exist or has been proposed to date.